The invention relates to inverters in general and more particularly to a method for shutting down an inverter with bridge arms with controlled main valves, and further bridge arms with recovery valves, the control terminals of which are connected via an enabling unit to a control set, to which a control voltage determining the firing angle of the firing pulses is fed. The invention further relates to a circuit for implementing the method according to the invention.
Inverters of the aforementioned type are described in U.S. Pat. Nos. 3,999,078 and 4,020,360 and in U.S. Applications Ser. No. 617,157. In these known inverters, the control device, which contains a controller and a control set, is connected to the control terminals of the controlled valves of the inverter via an enabling unit. Measures are taken so that a control voltage which corresponds to the desired output voltage of the inverter at any starting instant is already fed to the control set in a standby position. In the standby position, the control set already generates firing pulses for the inverter which, however, are blocked by the enabling unit. Upon a start command, the enabling unit is switched to pass the firing pulses. This makes it possible for the inverter to deliver the desired output load power particularly fast.
In many applications of inverters, the problem of shutting down the output stage of an inverter as fast as possible also arises. This is the case, for instance, in so-called converter drives, where a rotating field machine is fed via a frequency changer which consists of a rectifier, an intermediate link and an inverter. In the event of a failure of the rectifier supply, a defect in the rectifier or the blowing of a fuse in the d-c inputs to the inverter, a rapid shutdown of the inverter may become necessary. In interruption free power supplies with inverters of the type mentioned at the outset, it is also desireable to shut down the output stage of the inverter as quickly as possible upon the return of the line voltage, in order to prepare the inverter and the switching means for a new fast start.
Shutting down inverters is customarily accomplished by switching off the d-c supply and discharging the smoothing means and filter circuits following the inverter. During this process, the controlled valves of the inverter continue to receive timing pulses. The d-c supply is switched off and the smoothing means and filter circuits are discharged by means of mechanical switching means. With this procedure, it takes a relatively long time before the firing pulses for the controlled valves of the inverter can be switched off.